The effects of Klebsiella penumoniae infection on red blood cell and liver riboflavin levels and a few flavin enzymes were examined to verify our earlier hypothesis based on human experience that upper respiratory infections result in mobilization of riboflavin from tissues into blood, resulting in a rise in red blood cell riboflavin and an increased saturation of erythrocyte glutathione reductase (EGR) with its coenzyme flavin adenine dinucleotide (FAD; reduction in EGR activation coefficient, AC, values). Thirty-six-day-old male mice fed a diet marginally sufficient in riboflavin were injected with a single sublethal dose of K. pneumoniae. Batches of control, deficient infected and deficient uninfected animals were killed during the peak period of infection and after recovery. Infection brought about a significnat rise in basal EGR activity due to greater saturation with coenzyme FAD and a concomitant reduction in EGR-AC values. On recovery, basal EGR activity decreased, and EGR-AC values increased to levels beyond control values, suggesting riboflavin deficiency. In the liver, acylcoenzyme A dehydrogenase showed a significant decrease and pyridoxaminephosphate oxidase activity a significant increase. Both the enzymes normalized after recovery. D-amino acid oxidase activity did not change.